Magnetron output coupler



Oct. 2, 1956 P. w. CRAPUCHETTES 2,765,423

MAGNETRON OUTPUT COUPLER Filed Dec. 18, 1950 2 Sheets-Sheet 1 NVENTOR PAUL CFAPUCHETTES BYMW ATTO R N EY P. w. CRAPUCHETTES 2,765,423

MAGNETRON OUTPUT COUPLER Oct. 2, 1956 2 SheetsSheet 2 Filed Dec. 18, 1950 INVENTOR PAUL ML CRAPUCHETTES ATTORNEY U nitecl States Patent MAGNETRON OUTPUT COUPLER Paul W. Crapuchettes, Palo Alto, Calif., assignor to Litton Industries Inc.

Application December 18, 1950, Serial No. 201,471

7 Claims. (Cl. 315-39) This invention relates to magnetron output couplers and more particularly to couplers for matching the impedance of magnetron resonators to an output wave guide.

The problem of securing the proper impedance matching between a magnetron and an output wave guide is one which has concerned the art for a considerable period of time. The magnetron cavity resonators are generally of rather low impedance and the impedance decreases with increasing frequency of operation. On the other hand the impedance of a wave guide of the particular proportions is substantially constant. Accordingly, with increasing frequency of operation the problem of securing correct impedance match between the resonator and wave guide becomes increasingly difficult. Several general forms of impedance matching have been proposed and used in the past. The more satisfactory solution, particularly for tunable magnetrons is the provision of a so-called output ramp which comprises a set of plates or vanes within the end of the wave guide adjacent the magnetron. These vanes have their ends adjacent the magnetron relatively closely spaced and communicate with the resonator in the magnetron through an opening in the outer wall of the resonator. From this point on they taper exponentially to the wave guide walls. This type of coupling is quite satisfactory for frequencies wherein the opening from the magnetron is not too small. However, the initial loading in the magnetron must be decreased with increasing operating frequency so that this initial loading will vary inversely as the square root of the number of resonators divided by the wave length. For a magnetron working in the neighborhood of from 9-10 kilomegacycles the resonator impedance may be in the order of 2.5 ohms whereas a 2x1 rectangular wave guide has a substantially constant impedance of about 300 ohms. To achieve a match between these 'impedances by the tapered ramp requires a spacing at the magnetron output of .001 inch. This is so small that the heating of the magnetron with consequent distortion of parts and warping may completely upset the operation of the system.

A second type of system which has been used for impedance matching is the use of an effective single step one quarter wave impedance transformer. Such an impedance transformer however is inherently narrow band, especially when coupling into a magnetron which has only 2.5 ohms impedance. Furthermore, such a coupling at these higher frequencies does not provide a sufficient attenuation of the adjacent operating modes of the magnetron to assure good operation in the 1r mode. The impedance of the wave guide varies in precisely the wrong direction since it decreases with increase in frequency. Inherently the quarter wave impedance transformer inverts the frequency variation and tends to increase the loading in undesired modes. However, as previously stated, this is not sufiicient in itself to produce the desired broad band matching in the output coupling circuits.

In an effort to overcome these disadvantages I have found that a practical dimensioning of the output conpling slot can be made and a broad band impedance match achieved by providing within the wave guide an impedance matching unit wherein vanes similar to those used in the simple ramp impedance transformer are provided. However, the first section of these vanes adjacent to the magnetron output slot are made substantially a quarter wave length long so as to constitute a quarter wave length impedance transformer and from the end away from the magnetron resonator they are made to follow an exponential taper. With such a construction it has been demonstrated that at operations in the neighborhood of-9,000 megacycles a slot of .01 inch may be used and that a loading of undesired modes which increases 25% with an increase of 6% in frequency can be achieved. Moreover, it has been found that with this type of coupling impedance matching over a frequency band of about 800 megacycles can be achieved.

It is accordingly an object of my invention to provide a dual impedance output transformer comprising essentially a quarter wave impedance transformer and an exponential impedance transformer in tandem.

According to a feature of my invention I provide, for coupling a cavity resonator operating at an ultra high frequency to a wave guide, an impedance transformer unit comprising a pair of vanes within the wave guide and substantially in contact with the wave guide wall having their ends adjacent the cavity resonator spaced relatively closely together and extending rectilinearly at substantially constant spacing for a distance of substan' tially a quarter wave length at the mid operating frequency of the resonator and extending from the outer end of these rectilinear portions substantially exponentially to the resonator wall.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood taken in conjunction with the accompanying drawings, in which:

Figure 1 illustrates a cross-sectional view through a multi-cavity magnetron anode and the output wave guide coupler illustrating the construction of the output coupler in accordance with my invention.

Figure 2 is a cross-sectional view of a portion of Fig. 1 taken along the line 2-2 of Fig. l and Figure 3 is an illustration in section of a portion of the magnetron showing a modification of the output coupling slot shown in Fig. 1.

Turning now to Fig. l, the magnetron may comprise a cathode I mounted centrally of a plurality of cavity resonators 2. The resonators 2 may be constructed by providing a plurality of radial vanes 3 fastened at one end to the outer wall 4 of the magnetron. In actual construction a further body portion 5 is provided to support the resonator portions and to complete the tube. The vanes 3 are normally strapped together at alternate vanes to secure the desired operation and reduce mode jump-- ing. However, since such rings are not important to an understanding of the present invention they are omitted from the drawings in the interest of simplicity. Furthermore, the energizing circuits in the magnetic field producing means are likewise omitted.

One of the resonators 2 is coupled through an opening or slot 6 to a Wave guide 7. Wave guide 7 may be of any desired form and is illustrated herein as being rectangular in crosssection as will be apparent by reference to Fig. 2. At the outer end of the wave guide 7 is provided a glass or dielectric window 8 which may be sealed to support ring 9 and to the Wave guide 7 by means of an additional sealing ring, so that the interior of the wave guide may be evacuated. A window is provided to permit electromagnetic waves to pass on the other output circuits. Within wave guide 7 are provided 2 impedance transformer vanes and 11. The inner ends 12 and 13 of the vanes 10 and 11 are spaced apart a small distance determined by the frequency of operation desired. This spacing need. not be the same narrow spacing as would be required were the vanes a simple exponential transformer. These portions 12 and 13 extend from the wall 4 of the magnetron a distance sub? stantially equal to the quarter wave length of the wave within the guide (kg). From the outer ends of portions 12 and 13 extend the exponentially tapered portions 14 and 15 respectively of vanes 10 and 11. A small step or shoulder 16, 17 may be provided at the junction point between portions 12, 14; 13, 15 respectively. These shoulders however need not be provided but are used only as a matter of convenience in securing the desired end efiect on the quarter wave transformer section.

The portions 12, 13 constitute essentially a quarter wave length impedance transformer between the slot 6 and the wave guide 7 followed essentially in tandem by the exponential transformer defined by the portions 14 and 15 of vanes 10 and 11. With this construction impedance match between a cavity resonator and a Wave guide has been provided which has a substantial impedance match over 800 megacycles and a standing wave ratio no worse than 1.6 at the extremities of the point.

In the construction shown in Figs. 1 and 2 it will be noted that the slot 6 is directly in line with window 8. With such a construction it is found that stray electrons may be ejected through the opening and may puncture window 8. This may be avoided, for example by using a modified construction for the magnetron slot as shown in Fig. 3. in this illustration the slot 6 is made at an angle to the longitudinal axis of wave guide 7 so that electrons will tend to strike the wall of the Wave guide instead of impinging upon the output window. In other respects the illustration of Fig. 3 is substantially similar to that of Fig. 1 except that here there is no shoulder provided between the portions 12, 14 and 13, 15 of the impedance transformer vanes 10 and 11.

While I have described my invention with reference to magnetrons operating in the region of 9,000 megacycles and higher, it is clear that the principles of' the invention are applicable to magnetrons of lower frequency as well. Furthermore, although 1 have illustrated a magnetron in which the resonators are formed by radial vanes connected to an outer wall it is clear that the principles of my invention are applicable to other forms of magnetrons as well as to other types of circuits wherein impedance coupling between high frequency resonators and wave guides is desired.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

What is claimed is:

1. An impedance matching coupler system comprising a low impedance resonator, a high impedance wave guide wherein the coupling is effected between said resonator and wave guide by an opening from the resonator into the wave uide, and a coupler designed to match the impedance of said resonator and wave guide over a given frequency band, comprising a pair of impedance transformer vanes within said Wave guide extending from diametrically opposed inner surfaces thereof toward one another, the adjacent edges of said vanes having a first portion providing a rectilinear substantially parallel spacing extending from said opening for a distance substantially a quarter wave length at the mid-operating frequency of said frequency band, and a second portion wherein the adjacent edges of said waves provide a gradually increasing spacing to said outer surfaces of said wave guide.

2. A coupler according to claim 1, wherein the spacing of said second portion increases substantially in an exponential curve.

3. An impedance matching coupler according to claim 1 wherein said resonator is a cavity resonator of a magnetron, and wherein the Wave guide within which said impedance transformer vanes is mounted is evacuated.

4. An impedance matching coupler according to claim 3, wherein the opening between said resonator and said wave guide is made at an angle to the axis of said wave guide.

5. A magnetron comprising a wall portion, and a plurality of magnetron vanes extending from said wall to define separate magnetron cavity resonators surrounding a cathode, a wave guide sealed at one end to said wall, said wall portion having an opening communicating between one of said resonators and the interior of said wave guide, a pair of impedance transformer vanes mounted within said wave guide and extending from diametrically opposed surfaces of said wave guide toward each other, the inner edges of said vanes having a first portion substantial-ly a quarter wave length long at the operating frequency of said magnetron extending parallel to each other from said opening along said wave guide, and a second portion extending along said wave guide and gradually diverging in an exponential curve to said opposed surfaces.

6. A magnetron comprising a cylindrical wall portion, and a plurality of magnetron vanes extending radially inwardly from said wall to define separate magnetron cavity resonators surrounding a cathode, a wave guide sealed at one end to said wall and closed at its other end by a dielectric window, said wall portion having a longitudinal slot communicating between one of said resonators and the interior of said wave guide, a pair of impedance transformer vanes mounted within said wave guide and extending from diametrically opposed surfaces of said wave guide toward each other, the inner edges of said vanes having a first portion substantially a quarter wave length long at the operating frequency of said magnetron extending parallel to each other from said opening along said wave guide, and a second portion extending along said wave guide and gradually diverging in an exponential curve to said opposed surfaces.

7. A magnetron comprising a cylindrical wall portion, and a plurality of magnetron vanes extending radially inwardly from said wall to define separate magnetron cavity resonators surrounding a cathode, a wave guide sealed at one end to said wall and closed at its other end by a dielectric Window, said wall portion having a longitudinal slot communicating between one of said resonators and the interior of said wave guide, said slot being at an angle to the longitudinal axis of said wave guide, a pair of impedance transformer vanes mounted within said wave guide and extending from diametrically opposed surfaces of said wave guide toward each other, the inner edges of said vanes having a first portion substantially a quarter wave length long at the operating frequency of said rnagnetron extending parallel to each other from said opening along said wave guide, and a second portion extending along said wave guide and gradually diverging in an exponential curve to said opposed surfaces.

References Cited in the file of this patent UNITED STATES PATENTS 2,283,935 King May 26, 1942 2,412,772 Hansell Dec. 17, 1946 72,429,291 Okress Oct. 21, 1947 2,455,952 Schmidt Dec. 14, 1948 2,466,922 Wax Apr. 12, 1949 2,473,724 Okress et al June 21, 1949 2,512,901 Litton June 27, 1950 2,555,349 Litton June 5, 1951 

